Question

Why was iodine able to diffuse across the membrane while starch was not?

Answer 1

Starch cannot diffuse across the membrane because starch molecules are too large to fit through the pores in the dialysis tubing, whereas iodine is a smaller molecule and therefore can diffuse across.  

Answer 2

The correct reason would be their size difference iodine are small while starch molecules are large to pass through the membrane.

Explanation:

The permeability of the membrane also depends on the size of the molecule. The large molecule can not pass even though all other conditions are in their favor. For example Iodine molecules are small enough to go through the membrane freely, however starch molecules are too large and complex to pass through the membrane.

Thus, The correct reason would be their size difference iodine are small while starch molecules are large to pass through the membrane.